1. Field of the Invention
This invention relates to a process of producing an exposure mask, and more particularly to a process of producing an exposure mask wherein exposure processing and selective etching processing are performed for a mask film formed on a transparent substrate to form a mask pattern of the mask film.
2. Description of the Related Art
When semiconductor integrated circuits such as ICs, LSIs or VLSIs are to be produced, it is necessary to form various patterns on semiconductor substrates. Formation of the patterns is performed in accordance with an optical transfer method using a mask wherein a mask film is selectively formed on a transparent substrate that has been prepared in advance and then light, an ion beam, an electron beam or the like is projected onto a semiconductor substrate via the mask, as disclosed, for example, in Japanese Patent Laid-Open No. 59-178726.
More particularly, a photo-resist film of the negative type or of the positive type is first applied to a surface of a semiconductor substrate, and then, light, an ion beam, an electron beam or the like is projected onto the photo-resist film through the exposure mask. Then, where the photo-resist film is of the negative type, a portion of the photo-resist film which is irradiated with light cures, but alternatively where the photo-resist film is of the positive type, a portion of the photo-resist film which is irradiated with light becomes dissolvable. Afterwards, developing processing is performed. Consequently, where the photo-resist film is of the negative type, a remaining portion of the photoresist film other than the portion which has been irradiated to cure with light is removed, but alternatively where the photo-resist film is of the positive type, the portion of the photo-resist film which has been irradiated with light is removed while the other portion which has not been irradiated with light remains as it is. After such development, a predetermined pattern is formed on the semiconductor substrate by etching of a ground film and so forth using the photo-resist film as a mask. Further, introduction of impurities may sometimes be performed using the photo-resist film as a mask.
By the way, formation of a pattern on a semiconductor substrate is performed several times, and during such pattern formation, a photo-resist film of the negative type may be used or a photo-resist film of the positive type may be used, but all of the patterns are not formed using only one of the two types of photo-resist film. This is because, where the ratio of the area to be etched to the entire area of the semiconductor substrate is very low such as when a through-hole is formed in an insulator film, fine formation can be performed comparatively readily if a photo-resist film of the positive type is used, but on the other hand where the ratio of the area to be etched to the entire area of the semiconductor substrate is comparatively high, it is preferable to use a photo-resist film of the negative type. Besides, during the process of producing a semiconductor device, it is necessary to form a pattern wherein a portion to be etched is greater in area than the other portion which is not to be etched, and also it is necessary to form another pattern wherein the relationship is reversed.
FIG. 2(A) shows a negative reticule which is applied as an exposure mask for use in etching using a photo-resist film of the positive type, and FIG. 2(B) shows a positive reticule which is applied as an exposure mask for use in etching using a photo-resist film of the negative type.
Referring to FIGS. 2(A) and 2(B), the positive and negative reticules shown include each a transparent substrate a made of glass, and a mask film b formed on one of two opposite main surfaces of the transparent substrate a and commonly made of chrome. The negative reticule shown in FIG. 2(A) is normally produced in the following manner. First, a mask film b of chrome or the like is formed on an entire main surface of the transparent substrate a, and then a photo-resist film of the positive type is formed on the mask film b and then exposed to light using an electron beam exposing device. Then, the photo-resist film of the positive type is developed, whereafter the mask film b is etched using the photo-resist film of the positive type as a mask. In this instance, the ratio of the area of the remaining mask film b to the area of the entire reticule is considerably higher than 50%. On the other hand, the positive reticule shown in FIG. 2(B) is produced using a photo-resist film of the negative type as a resist film and is different in that the ratio of the area of the mask film b remaining on the transparent substrate 1 after selective etching to the entire area of the reticule is considerably lower than 50%. Conventionally, however, such positive reticule as well as the negative reticule are produced by the same process.
By the way, as higher density integration of semiconductor devices proceeds, it becomes progressively more important to raise the accuracy in registration of layers formed by chip pattern transfer. Therefore, it is required to improve the accuracy in registration between individual reticules used for the production of a device. However, actually it is difficult to satisfy such requirement.
Thus, an investigation of the cause of such registration inaccuracies has been conducted. From the investigation, it has become apparent that the accuracy in position is different between a reticule wherein the ratio of the area occupied by the mask film b to the entire area of the reticule is comparatively high as in the case of the reticule shown in FIG. 2(A) and another reticule wherein the ratio of the area occupied by the mask film b to the entire area of the reticule is comparatively low as shown in FIG. 2(B), and as a result, an error in registration occurs between a layer, a film, a through-hole or the like formed using such a negative reticule as shown in FIG. 2(A) and a layer, a film, a through-hole or the like formed using such a positive reticule as shown in FIG. 2(B). These registration errors are a significant obstruction to further higher integration of semiconductor devices. Further, it has become apparent that the difference in accuracy in position between a positive reticule and a negative reticule arises from the fact that they are subject to differences in the degree to which they are warped by thermal stress. This will be described more in detail below with reference to FIGS. 3 and 4.
At a stage before a mask pattern is formed but after a mask film b made of chrome has been formed, for example, by vapor deposition on an entire surface of a transparent substrate a made of glass, the exposure mask has some degree of warp arising from thermal stress as shown at c in FIG. 3, wherein such warp is shown in a somewhat exaggerated manner. Photo etching is then performed for the mask film b of the exposure mask c to form a negative reticule such as, for example, shown at d in FIG. 3 or a positive reticule such as shown at e in FIG. 3. In this instance, since the area of the mask film b is reduced upon such photo etching, also the degree of warp by thermal stress is reduced. However, the degree of reduction of such warp by thermal stress is different between the positive reticule and the negative reticule, and the degree of reduction of the warp of the positive reticule wherein the area of the mask film b is comparatively small is greater than that of the negative reticule wherein the relationship is reverse. As a result, the degree of displacement in position coordinates which is caused by reduction of the warp is smaller with the negative reticule than with the positive reticule as seen from FIGS. 4(A) and 4(B). In particular referring to FIGS. 4(A) and 4(B), solid lines indicate ideal gratings while broken lines indicate actual gratings after etching, and a displacement between the gratings is a displacement of coordinates caused by warp. Then, if articles having substantially the same ratios of the area of the mask film b to the entire area of the reticule can be used at all of the steps, then the tolerance in registration of layers described above can be reduced to a very small value. Actually, however, it is impossible to form all mask patterns using reticules having ratios of a substantially same level of the mask film b area to the entire area of the reticule, and consequently, it cannot be avoided that the ratio of the mask film b area to the entire area of the reticule is different for each mask pattern forming step. Accordingly, there is a great limitation to improving the layer registration accuracy using conventional techniques.